Stands for "Prolonged Rocket Boost", which is a very advanced racing technique used in the game Mario Kart DS for the Nintendo DS that allows the driver to go off-road while still maintaining maximum speed.
There are two ways to achieve PRB:
1. Get a perfect start boost and snake perfectly (good enough so that your kart is constantly at full maximum speed). Continue to snake perfectly onto the off-road section you wish to cut across.
2. Snake perfectly after leaving a rainbow zipper without ever slowing down.
Any loss in speed whatsoever or use of a mushroom will end the PRB. The Karts best built for PRB are the Dry Bomber and Egg1, however it is possible with many more, such as the Standard DS, Standard YS, ROB-BLS, Poltergust 4000, and even more if you've got ultra skills in the game.
I just did 3 full laps of PRB on SNES Mario Circuit 1!!!!